[Extensions Docs] Update the mac shortcut in a sample extension

Mac now has a default shortcut for "Command+Shift+Y", so update
the commands sample extension to use "MacCtrl+Shift+Y" to avoid
conflict.

Bug: None
Change-Id: Iced6c9bc9dc43f99461cc0c3bd93eaa19da1d6a1
Reviewed-on: https://chromium-review.googlesource.com/748025Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515973}

chrome/common/extensions/docs/examples/api/commands/manifest.json

 {
   "name": "Sample Extension Commands extension",
-  "description": "Press Ctrl+Shift+F (Command+Shift+F on a Mac) to open the browser action popup, press Ctrl+Shift+Y to send an event (Command+Shift+Y on a Mac).",
+  "description": "Press Ctrl+Shift+F to open the browser action popup, press Ctrl+Shift+Y to send an event.",
   "version": "1.0",
   "manifest_version": 2,
   "background": {
@@ -12,7 +12,10 @@
   },
   "commands": {
     "toggle-feature": {
-      "suggested_key": { "default": "Ctrl+Shift+Y" },
+      "suggested_key": {
+        "default": "Ctrl+Shift+Y",
+        "mac": "MacCtrl+Shift+Y"
+       },
       "description": "Send a 'toggle-feature' event to the extension"
     },
     "_execute_browser_action": {